This invention relates to laser drivers, and more particularly, this invention relates to laser drivers used for driving an optic preamplifier.
The bandwidth of single channel (or wavelength) fiber optic telecommunication links is mainly limited by the high-speed electronics required at the transmitter and receiver. Wavelength division multiplexing of optical communication signals is a technique used for increasing the bandwidth of a fiber optic telecommunications link, without increasing the speed of the electronics. At the communications receiver, the optical channels that receive optical communication signals must be separated, or demultiplexed, and sent to their individual receivers, which vary in their rate of data receipt. One example is 2.488 Gb/s receivers.
The demultiplexing process is not ideal and optical losses are incurred, thus reducing the overall receiver sensitivity. A reduction in sensitivity also translates into shorter transmission lengths for the overall telecommunications link. When components are optimized on an individual basis, the benefits of any smaller size and lower power operation are not achieved with these type of receiver architectures. One current method of achieving high sensitivities in a wavelength division multiplexed receiver is the use of a wavelength demultiplexer with avalanche photodiodes (APD). These electronically amplified optical receivers have been designed as separate units in a rack-mounted configuration. Typically, each card unit within a rack-mounted configuration represents an individual component, forming a very large, but undesirable unit, especially in low power applications, as in advanced aircraft designs or other design specifications where low power and small footprint are desired.
Because these types of optical receivers are rack-mounted units and use avalanche photodiodes, the receiver sensitivity power penalty is incurred approximately equal to the optical insertion loss of the optical demultiplexer. Typically, telecommunications receivers using optical pre-amplification are not optimized for both high sensitivity and low power, and are not contained within a single assembly. Also, in some optical communication receivers, a laser driver may be necessary. To deliver the current necessary to power a laser diode, an electric circuit is used and supplies power to the laser driver, but also dissipates power of its own. This power, which is dissipated in the control circuit, is essentially wasted power, because it is not converted into photons.
Some current design injection laser diode drivers use a linear pass transistor to deliver a regulated current to the injection laser diode. This method results in a constant voltage across the device and constant current through the device, resulting in a large amount of dissipated power. For example, nearly 90% of all power dissipated by the injection laser diode driver occurs in the pass transistor, in some prior art designs. Thus, there is a necessary requirement and solution desired to deliver a clean current source to the injection laser diode.
The present invention is advantageous and provides a low power laser driver that drives an optically amplified preamplifier for low power applications, such as advanced commercial and military aircraft, where lightweight and low power are required.
In accordance with the present invention, the laser driver includes an injection laser diode and a current source control loop circuit connected to the injection laser diode for establishing a fixed current through the injection laser diode. A voltage switcher circuit is connected to the injection diode and current source control loop circuit. This voltage switcher circuit is adapted to receive a fixed supply voltage and convert inductively the supply voltage down to a forward voltage for biasing the laser diode and producing an optical output having minimized power losses.
A high efficiency current source is connected to the laser diode. The voltage switcher circuit is monolithically formed as a single circuit chip. The current source control loop circuit also includes a current source leg having at least one electronic component. This voltage switcher circuit has an output that is varied such that there is minimal voltage drop across the at least one component of the current source leg. In one aspect of the invention, the injection laser diode comprises a high quantum efficiency laser diode. The fixed supply voltage is about five volts as used with semiconductor technology.
The laser driver circuit formed by the injection laser diode, current source control loop circuit and voltage switcher circuit form an integrated laser driver that is received within a laser driver housing. In yet another aspect of the present invention, an external Bragg grating is operatively connected to the injection laser diode and receives the optical output and stabilizes the optical wavelength of the output. This external Bragg grating is used to eliminate a thermal electric cooler.